The subject of the present invention is a combined propulsion system intended for a spacecraft.
Two main techniques employing electrical energy are currently known and can be used by way of propulsion system for a spacecraft, for example to alter the orbit of a satellite.
The first technique, known as electric propulsion, employs a propulsion fluid which is heated or accelerated using an electric current. This technique can be used in different ways, in order to achieve high propulsion fluid expulsion speeds, and these fall under three main categories:
1) Electrothermal propulsion: the propulsion fluid is electrically heated and expanded in a nozzle.
2) Electrostatic propulsion: the ionized particles of the propulsion fluid are accelerated by an electric field. Such a system is described for example in U.S. Pat. No. 5,947,421 (BEATTIE) and U.S. Pat. No. 6,195,980 (WALTHER).
3) Electromagnetic propulsion: a current induced in a plasma of the propulsion fluid interacts with an internal or external magnetic field to generate an acceleration force along the axis of the ejected stream. Such a system is described in patent U.S. Pat. No. 6,293,090 (OLSON).
Such systems make it possible to considerably increase the exit speeds of the propulsion fluid, by comparison with chemical rocket motors, this increase being of about one order of magnitude, making them highly advantageous for space missions.
These techniques do, however, have the disadvantage of requiring that electrical energy be available and their thrust density (defined as the thrust per unit area of the exit of the propulsion nozzle) is markedly lower than that obtained with chemical rocket motors.
The second technique employs an electrodynamic tether which is in the form of a very long (typically several hundred meters long) conducting wire which extends from a spacecraft, for example a satellite. The gravity gradient creates a force (known as the xe2x80x9ctidal forcexe2x80x9d) which orients the tether in a vertical direction. If, for example, the tether is attached to a satellite in orbit around the Earth, it crosses the lines of the Earth""s magnetic field at a speed equal to the orbital speed of the satellite, for example 7 to 8 km/s in the case of a satellite in a medium orbit. The movement of the tether through a magnetic field induces a potential difference which may be of the order of several hundreds of volts per kilometer length.
If the system is equipped with a device for collecting electrons from the plasma in the ionosphere at one end of the tether and for expelling these electrons at the other end of the tether, then the tether acts as a brake. Such a system is described in the article by Robert P. HOYT and Robert L. FORWARD entitled xe2x80x9cPerformance of the Terminator Tether for Autonomous Deorbit of LEO Spacecraftxe2x80x9d, published in 1999 under the reference AIAA99-2839, pages 1 to 10 by the American Institute of Aeronautics and Astronautics.
The voltage generated creates a current along the tether and this current J, by interacting with the Earth""s magnetic field B, generates a Lorentz force F equal to the product J.B which has the effect of slowing the satellite+tether assembly and the electrodynamic force thus created lowers the orbit of the satellite+tether assembly. The energy balance for the operation is that the tether converts the orbital energy of the spacecraft into electrical energy which is dissipated by thermal heating of the tether.
In acceleration mode (see, for example, patent U.S. Pat. No. 4,824,051) the tether is used to accelerate the spacecraft and therefore increase the altitude of its orbit. A device of this type is also described in the article by John H. BLUMER and collaborators, entitled xe2x80x9cPracticality of using a Tether for Electrodynamic Reboost of the International Space Stationxe2x80x9d, published in 2001, page 445 to page 451 in xe2x80x9cSpace Technology and Applications Forumxe2x80x942001xe2x80x9d, published by M. S. El GENKxe2x80x94American Institute of Physics. For this, an electric generator is coupled to the tether to force an electric current in the opposite direction to the normal direction of flow corresponding to braking and the current generates a Lorentz acceleration force which increases the orbit of the spacecraft. This propulsion technique has the advantage of not requiring the ejection of a propulsion fluid, but the disadvantage of being sensitive to irregularities in the magnetic field.
The aforementioned patent U.S. Pat. No. 6,293,090 suggests the simultaneous but independent use of electric propulsion and a tether system before the spacecraft is taken to a higher orbit.
The subject of the present invention is a propulsion system intended for a space vessel, which combines a tether propulsion system and an electric propulsion system.
The idea underlying the invention is to combine an electrodynamic tether propulsion system and an electrostatic propulsion system.
Thus the invention relates to a combined propulsion system for a spacecraft, combining an electric propulsion system and a tether propulsion system, characterized in that the electric propulsion system is an electrostatic system which has a generator for ejecting an ionized plasma, comprising at least one grid for electrostatically accelerating the ions in the plasma and, on the other hand, an electron generator generating electrons to maintain the electrical neutrality of the system, and in that the electron generator is coupled to a first end of the tether which is the opposite end to a second end at which the plasma-ejection generator is arranged, generating a compensating current flowing along said tether.
As the electron generator is coupled to the end of the tether which is the opposite end to the end at which the device for ejecting ionized plasma that provides electrostatic propulsion is arranged, the neutralizing current flows through the tether and this current, with the magnetic field with which the spacecraft is surrounded, produces a Lorentz force which accelerates the satellite, enhancing the propulsive action of the electrostatic system.
The system may be characterized in that the plasma-ejection generator is arranged on the spacecraft and in that the electron generator is coupled to a free end of the tether which is the opposite end to the end at which the tether is attached to the spacecraft and electrically coupled thereto.
According to another alternative, it is characterized in that the electron generator is arranged on the spacecraft and in that the plasma-ejection generator is secured to a free end of the tether to which end it is electrically coupled, this free end being at the opposite end to the end at which the tether is attached to the spacecraft and is electrically coupled thereto.
The system may comprise a device for deploying the tether and the device (ion-ejection generator or electron generator) which is coupled to its free end.
Advantageously, the tether propulsion system comprises said tether, said electron generator and a device for deploying the tether and said electron generator which is coupled to said free end of the tether.